NAME

SYNOPSIS

DESCRIPTION

The ptrace() system call provides a means by which a parent process may
observe and control the execution of another process, and examine and
change its core image and registers. It is primarily used to implement
breakpoint debugging and system call tracing.
The parent can initiate a trace by calling fork(2) and having the
resulting child do a PTRACE_TRACEME, followed (typically) by an
exec(3). Alternatively, the parent may commence trace of an existing
process using PTRACE_ATTACH.
While being traced, the child will stop each time a signal is
delivered, even if the signal is being ignored. (The exception is
SIGKILL, which has its usual effect.) The parent will be notified at
its next wait(2) and may inspect and modify the child process while it
is stopped. The parent then causes the child to continue, optionally
ignoring the delivered signal (or even delivering a different signal
instead).
When the parent is finished tracing, it can terminate the child with
PTRACE_KILL or cause it to continue executing in a normal, untraced
mode via PTRACE_DETACH.
The value of request determines the action to be performed:
PTRACE_TRACEME
Indicates that this process is to be traced by its parent. Any
signal (except SIGKILL) delivered to this process will cause it
to stop and its parent to be notified via wait(2). Also, all
subsequent calls to execve(2) by this process will cause a
SIGTRAP to be sent to it, giving the parent a chance to gain
control before the new program begins execution. A process
probably shouldn’t make this request if its parent isn’t
expecting to trace it. (pid, addr, and data are ignored.)
The above request is used only by the child process; the rest are used
only by the parent. In the following requests, pid specifies the child
process to be acted on. For requests other than PTRACE_KILL, the child
process must be stopped.
PTRACE_PEEKTEXT, PTRACE_PEEKDATA
Reads a word at the location addr in the child’s memory,
returning the word as the result of the ptrace() call. Linux
does not have separate text and data address spaces, so the two
requests are currently equivalent. (The argument data is
ignored.)
PTRACE_PEEKUSER
Reads a word at offset addr in the child’s USER area, which
holds the registers and other information about the process (see
<linux/user.h> and <sys/user.h>). The word is returned as the
result of the ptrace() call. Typically the offset must be word-
aligned, though this might vary by architecture. (data is
ignored.)
PTRACE_POKETEXT, PTRACE_POKEDATA
Copies the word data to location addr in the child’s memory. As
above, the two requests are currently equivalent.
PTRACE_POKEUSER
Copies the word data to offset addr in the child’s USER area.
As above, the offset must typically be word-aligned. In order
to maintain the integrity of the kernel, some modifications to
the USER area are disallowed.
PTRACE_GETREGS, PTRACE_GETFPREGS
Copies the child’s general purpose or floating-point registers,
respectively, to location data in the parent. See
<linux/user.h> for information on the format of this data.
(addr is ignored.)
PTRACE_GETSIGINFO (since Linux 2.3.99-pre6)
Retrieve information about the signal that caused the stop.
Copies a siginfo_t structure (see sigaction(2)) from the child
to location data in the parent. (addr is ignored.)
PTRACE_SETREGS, PTRACE_SETFPREGS
Copies the child’s general purpose or floating-point registers,
respectively, from location data in the parent. As for
PTRACE_POKEUSER, some general purpose register modifications may
be disallowed. (addr is ignored.)
PTRACE_SETSIGINFO (since Linux 2.3.99-pre6)
Set signal information. Copies a siginfo_t structure from
location data in the parent to the child. This will only affect
signals that would normally be delivered to the child and were
caught by the tracer. It may be difficult to tell these normal
signals from synthetic signals generated by ptrace() itself.
(addr is ignored.)
PTRACE_SETOPTIONS (since Linux 2.4.6; see BUGS for caveats)
Sets ptrace options from data in the parent. (addr is ignored.)
data is interpreted as a bit mask of options, which are
specified by the following flags:
PTRACE_O_TRACESYSGOOD (since Linux 2.4.6)
When delivering syscall traps, set bit 7 in the signal
number (i.e., deliver (SIGTRAP|0x80) This makes it easy
for the tracer to tell the difference between normal
traps and those caused by a syscall.
(PTRACE_O_TRACESYSGOOD may not work on all
architectures.)
PTRACE_O_TRACEFORK (since Linux 2.5.46)
Stop the child at the next fork(2) call with SIGTRAP|PTRACE_EVENT_FORK<<8 and automatically start tracing
the newly forked process, which will start with a
SIGSTOP. The PID for the new process can be retrieved
with PTRACE_GETEVENTMSG.
PTRACE_O_TRACEVFORK (since Linux 2.5.46)
Stop the child at the next vfork(2) call with SIGTRAP|PTRACE_EVENT_VFORK<<8 and automatically start tracing
the newly vforked process, which will start with a
SIGSTOP. The PID for the new process can be retrieved
with PTRACE_GETEVENTMSG.
PTRACE_O_TRACECLONE (since Linux 2.5.46)
Stop the child at the next clone(2) call with SIGTRAP|PTRACE_EVENT_CLONE<<8 and automatically start tracing
the newly cloned process, which will start with a
SIGSTOP. The PID for the new process can be retrieved
with PTRACE_GETEVENTMSG. This option may not catch
clone(2) calls in all cases. If the child calls clone(2)
with the CLONE_VFORK flag, PTRACE_EVENT_VFORK will be
delivered instead if PTRACE_O_TRACEVFORK is set;
otherwise if the child calls clone(2) with the exit
signal set to SIGCHLD, PTRACE_EVENT_FORK will be
delivered if PTRACE_O_TRACEFORK is set.
PTRACE_O_TRACEEXEC (since Linux 2.5.46)
Stop the child at the next execve(2) call with SIGTRAP|PTRACE_EVENT_EXEC<<8.
PTRACE_O_TRACEVFORKDONE (since Linux 2.5.60)
Stop the child at the completion of the next vfork(2)
call with SIGTRAP|PTRACE_EVENT_VFORK_DONE<<8.
PTRACE_O_TRACEEXIT (since Linux 2.5.60)
Stop the child at exit with SIGTRAP|PTRACE_EVENT_EXIT<<8. The child’s exit status can be
retrieved with PTRACE_GETEVENTMSG. This stop will be
done early during process exit when registers are still
available, allowing the tracer to see where the exit
occurred, whereas the normal exit notification is done
after the process is finished exiting. Even though
context is available, the tracer cannot prevent the exit
from happening at this point.
PTRACE_GETEVENTMSG (since Linux 2.5.46)
Retrieve a message (as an unsignedlong) about the ptrace event
that just happened, placing it in the location data in the
parent. For PTRACE_EVENT_EXIT this is the child’s exit status.
For PTRACE_EVENT_FORK, PTRACE_EVENT_VFORK and PTRACE_EVENT_CLONE
this is the PID of the new process. Since Linux 2.6.18, the PID
of the new process is also available for
PTRACE_EVENT_VFORK_DONE. (addr is ignored.)
PTRACE_CONT
Restarts the stopped child process. If data is nonzero and not
SIGSTOP, it is interpreted as a signal to be delivered to the
child; otherwise, no signal is delivered. Thus, for example,
the parent can control whether a signal sent to the child is
delivered or not. (addr is ignored.)
PTRACE_SYSCALL, PTRACE_SINGLESTEP
Restarts the stopped child as for PTRACE_CONT, but arranges for
the child to be stopped at the next entry to or exit from a
system call, or after execution of a single instruction,
respectively. (The child will also, as usual, be stopped upon
receipt of a signal.) From the parent’s perspective, the child
will appear to have been stopped by receipt of a SIGTRAP. So,
for PTRACE_SYSCALL, for example, the idea is to inspect the
arguments to the system call at the first stop, then do another
PTRACE_SYSCALL and inspect the return value of the system call
at the second stop. (addr is ignored.)
PTRACE_SYSEMU, PTRACE_SYSEMU_SINGLESTEP (since Linux 2.6.14)
For PTRACE_SYSEMU, continue and stop on entry to the next
syscall, which will not be executed. For
PTRACE_SYSEMU_SINGLESTEP, do the same but also singlestep if not
a syscall. This call is used by programs like User Mode Linux
that want to emulate all the child’s system calls. (addr and
data are ignored; not supported on all architectures.)
PTRACE_KILL
Sends the child a SIGKILL to terminate it. (addr and data are
ignored.)
PTRACE_ATTACH
Attaches to the process specified in pid, making it a traced
"child" of the calling process; the behavior of the child is as
if it had done a PTRACE_TRACEME. The calling process actually
becomes the parent of the child process for most purposes (e.g.,
it will receive notification of child events and appears in
ps(1) output as the child’s parent), but a getppid(2) by the
child will still return the PID of the original parent. The
child is sent a SIGSTOP, but will not necessarily have stopped
by the completion of this call; use wait(2) to wait for the
child to stop. (addr and data are ignored.)
PTRACE_DETACH
Restarts the stopped child as for PTRACE_CONT, but first
detaches from the process, undoing the reparenting effect of
PTRACE_ATTACH, and the effects of PTRACE_TRACEME. Although
perhaps not intended, under Linux a traced child can be detached
in this way regardless of which method was used to initiate
tracing. (addr is ignored.)

RETURNVALUE

On success, PTRACE_PEEK* requests return the requested data, while
other requests return zero. On error, all requests return -1, and
errno is set appropriately. Since the value returned by a successful
PTRACE_PEEK* request may be -1, the caller must check errno after such
requests to determine whether or not an error occurred.

ERRORS

EBUSY (i386 only) There was an error with allocating or freeing a
debug register.
EFAULT There was an attempt to read from or write to an invalid area in
the parent’s or child’s memory, probably because the area wasn’t
mapped or accessible. Unfortunately, under Linux, different
variations of this fault will return EIO or EFAULT more or less
arbitrarily.
EINVAL An attempt was made to set an invalid option.
EIOrequest is invalid, or an attempt was made to read from or write
to an invalid area in the parent’s or child’s memory, or there
was a word-alignment violation, or an invalid signal was
specified during a restart request.
EPERM The specified process cannot be traced. This could be because
the parent has insufficient privileges (the required capability
is CAP_SYS_PTRACE); non-root processes cannot trace processes
that they cannot send signals to or those running set-user-
ID/set-group-ID programs, for obvious reasons. Alternatively,
the process may already be being traced, or be init(8) (PID 1).
ESRCH The specified process does not exist, or is not currently being
traced by the caller, or is not stopped (for requests that
require that).

CONFORMINGTO

SVr4, 4.3BSD

NOTES

Although arguments to ptrace() are interpreted according to the
prototype given, glibc currently declares ptrace() as a variadic
function with only the request argument fixed. This means that
unneeded trailing arguments may be omitted, though doing so makes use
of undocumented gcc(1) behavior.
init(8), the process with PID 1, may not be traced.
The layout of the contents of memory and the USER area are quite OS-
and architecture-specific.
The size of a "word" is determined by the OS variant (e.g., for 32-bit
Linux it is 32 bits, etc.).
Tracing causes a few subtle differences in the semantics of traced
processes. For example, if a process is attached to with
PTRACE_ATTACH, its original parent can no longer receive notification
via wait(2) when it stops, and there is no way for the new parent to
effectively simulate this notification.
When the parent receives an event with PTRACE_EVENT_* set, the child is
not in the normal signal delivery path. This means the parent cannot
do ptrace(PTRACE_CONT) with a signal or ptrace(PTRACE_KILL). kill(2)
with a SIGKILL signal can be used instead to kill the child process
after receiving one of these messages.
This page documents the way the ptrace() call works currently in Linux.
Its behavior differs noticeably on other flavors of Unix. In any case,
use of ptrace() is highly OS- and architecture-specific.
The SunOS man page describes ptrace() as "unique and arcane", which it
is. The proc-based debugging interface present in Solaris 2 implements
a superset of ptrace() functionality in a more powerful and uniform
way.

BUGS

On hosts with 2.6 kernel headers, PTRACE_SETOPTIONS is declared with a
different value than the one for 2.4. This leads to applications
compiled with such headers failing when run on 2.4 kernels. This can
be worked around by redefining PTRACE_SETOPTIONS to
PTRACE_OLDSETOPTIONS, if that is defined.